Batteries have been recently used for applications in the power sources of hybrid electric vehicles besides applications in conventional miniaturized electronic devices. This entails a desire for batteries having, for example, a high capacity, long-cycle life, and fast chargeability. Because the highest amount of active material is filled in a restricted space in a battery, the electrode is resultantly compressed in a higher density.
In the production of an electrode, an active material-containing slurry is applied to a current collector made of, for example, a metal foil, dried, and then, the coated portion is compressed by a roll press machine or the like. Although the current collector lying under the compressed coated portion is also stretched by plastic deformation, the uncoated portion which is not coated with the active material-containing slurry is not so much stretched as the coated portion because the pressure of the press machine is not applied to the current collector at the uncoated portion where the active material-containing slurry is not applied. As a result, a difference in the elongation of the current corrector between the coated and uncoated portions causes residual stress to act on the boundary between the coated portion and the uncoated portion, giving rise to the strain and warpage of the electrode.
When such an electrode is laminated on a separator and the laminate is wound, the distortion and warpage give rise to weaving and also cause wrinkles and cracks of the electrode and hence electrode breakage when compensating the weaving. Also, the distortion and warpage cause deteriorated quality and reduced yield and also cause a hindrance to high-speed operation of the production line. The reasons of the distortion and warpage of the electrode are a difference in the elongation of the current collector between the slurry-coated portion and the slurry-uncoated portion after compression. Proposed examples of a measures to solve the problem include a method in which a groove is formed on the press roll to compress the uncoated portion and the coated portion at the same time to stretch the uncoated current collector, and a method in which the current collector is further plastically deformed by tensile stress to stretch the current collector.
However, it is guessed that in the method forming a groove on a press roll, the surface of the roll which will be abraded by pressing the current collector needs to repolish at relatively high frequency and therefore, the use of the groove shape of the press roll is not effective technologically and efficiently. Moreover, when the uncoated current collector comes out of the groove due to, for example, meandering of the electrode, there is the problem concerning uneven compressive density and breakage of the electrode.
Examples of the method of plastically deforming the current collector by tensile stress to stretch the current collector include an application of the tension annealing treatment which is usually used in, for example, rolling and processing of iron steel materials. Tensile stress is applied to a material to be processed under heating, thereby producing such an effect as to reduce the stress required for elastic deformation. When, for example, an aluminum foil is used for the current collector, the standard of the stress required for plastic deformation is 100 N/mm2 or more though depending on the thickness of the current collector and aluminum purity. The stress required for plastic deformation can be remarkably decreased by heating the current collector. However, in the case where the active material-containing layer of the electrode contains a material or component which is likely to be denatured at high temperatures, thereby deteriorating the performance of the battery, it is inevitable to limit heating temperature. This results in that the tensile stress cannot be significantly reduced in the heating temperature range to be applied, so that a stress several times the tensile stress originally required for winding up the electrode is required, arousing a fear as to breakage of the electrode and deterioration in winding accuracy.